The present invention relates to Schottky barrier infrared imaging arrays and, more particularly, to such arrays having increased fill-factor and capable of X-Y addressable readout.
Schottky barrier photodiode arrays have become increasingly attractive for infrared imaging systems. A single cell or picture element (pixel) of such an array, e.g., comprises a Schottky photodiode including a thin (e.g., 100 angstrom) layer of a metal silicide such as platinum silicide (PtSi) or palladium silicide (PdSi) formed on a semiconductor substrate. A typical example is a P-conductivity type silicon substrate. The metal Schottky electrode thus comprises the photodiode cathode, and the P-type silicon comprises the anode. For sensing operation the diode is reverse biased, defining in the semiconductor material underneath the Schottky metal electrode a depletion region wherein majority carriers (i.e., holes) are depleted and a positive charge appears on the Schottky metal electrode. Infrared photons striking the Schottky metal electrode generate electronhole pairs and the excited holes cross the Schottky barrier (having a barrier height represented by .phi..sub.SB) into the silicon. The photodiode is thus, in effect, discharged as the depletion region decreases in volume and the excess positive charges on the Schottky metal electrode decrease in number.
For infrared imaging purposes, Schottky photodiode arrays have a number of inherent advantages and unique characteristics compared to other types of infrared imagers such as metal-insulator-semiconductor (MIS) capacitors or ordinary junction diodes. One fundamental advantage is that it is much easier to achieve response to infrared radiation (e.g., wavelength in the range of 3 to 10 microns) while still employing silicon integrated circuit fabrication processes which are well developed in the art. This advantage arises from the fact that the spectral response is determined primarily by the physics of the barrier transition process and not by the photoabsorption process. In particular, during operation of a Schottky photodiode, electron-hole pairs are generated in the metal silicide rather than in the semiconductor, and the cut-off wavelength, in microns, is a function of the Schottky barrier height .phi..sub.SB in accordance with the expression (1.24/.phi..sub.SB), where .phi..sub.SB is in electron volts. Barrier height, and thus spectral response, can be determined through such design choices as the particular metal employed and the semiconductor conductivity type.
In contrast, in a junction photodiode or in an MIS capacitor, electron-hole pairs are generated in the semiconductor material, and the cut-off wavelength, in microns, is determined in accordance with the expression (1.24/E.sub.g), where E.sub.g is the bandgap in electron volts. Silicon devices respond to visible wavelength photons, e.g., having a wavelength in the order of 0.7 microns. Where semiconductor-type imaging devices (e.g., MIS) are required to respond to infrared radiation, more exotic semiconductor materials must be employed, such as InSb or HgCdTe.
Another advantageous characteristic of Schottky barrier photodiodes when employed in arrays is that crosstalk between adjacent pixels inherently is substantially nonexistent and channel isolation between adjacent pixels is not required. This characteristic arises from the fact that the electrons, which represent the signal generated by the incident radiation, remain in the Schottky metal electrode so that the location of the electrons is well-defined.
Nevertheless, the advantageous characteristics of Schottky diode photodetectors have not heretofore been fully realized in arrays. Schottky photodiode arrays have previously been read out employing such elements as transfer gates and various forms of charge coupled devices (CCD's). The CCD's are typically arranged in shift-register fashion to serialize readout data from an entire row or column of the photodiode array. However, the presence of these additional elements for reading out the Schottky photodiodes makes somewhat less than 30% of the chip area available for photon detection. This factor, referred to in the art as the "fill-factor", is the ratio of actual detection area to the total chip area occupied by the array. Moreover, it is difficult to place the individual detection sites as close together as may be required for desired resolution. As a result, the advantageous characteristics of substantially no crosstalk and no requirement for channel isolation are not effectively utilized. These advantageous characteristics can be fully utilized only if the readout structure permits a closely-spaced array, but this has not been possible in the past.
A Schottky photodiode has relatively low quantum efficiency, and it is important for this reason also to use efficiently all the area available. A conflicting requirement with a CCD readout is that the CCD must be made large enough to handle the high infrared background charge often encountered. In addition, because a large number of charge transfers is required in a large area array, a buried channel CCD is normally preferred. However, the low charge handling capacity of the buried channel CCD requires a larger area of the silicon chip and hence reduces the fill-factor.
By way of example, Schottky barrier infrared detector arrays are disclosed in the following documents: Roosild et al., U.S. Pat. No. 3,902,066; B. R. Capone et al., "Design and Characterization of A Schottky Infrared Charge-Coupled Device (IRCCD) Focal Plane Array", SPIE vol. 267--Staring Infrared Focal Plane Technology, pp. 39-45 (1981); M. Kimata et al., "Platinum Silicide Schottky-Barrier IR-CCD Image Sensors", Proceedings of the 13th Conference on Solid State Devices, Tokyo, 1981; Japanese Journal of Applied Physics, vol. 21 (1982), Supplement 21-1, pp. 231-235; and M. Cantella et al., "Solid State Focal Plane Arrays Boost IR Sensor Capabilities", Military Electronics/Countermeasures, September 1982, pp. 38-42.
AC-coupled (capacitive) readout of PN junction photodiode arrays responsive to visible light has also previously been proposed. Exemplary disclosures are: P. K. Weimer et al., "Multielement Self-Scanned Mosaic Sensors", IEEE Spectrum, March 1969, pp. 52-65; and E. Arnold et al., "New Solid-State Imaging Array with Reduced Switching Noise", 1971 IEEE International Solid-State Circuits Conference, Feb. 18, 1971, pp. 128-129. Further, some imagers employing MIS capacitors include two MIS capacitors at each sensing site with the two capacitor electrodes connected directly and respectively to row and column lines for X-Y addressing. The two MIS capacitors are coupled together so that stored charge can be transferred from one capacitor to the other. For example, see G. J. Michon, U.S. Pat. No. 3,786,263, and Michon et al., U.S. Pat. No. 3,085,062, both assigned to the instant assignee.
Despite the relatively early suggestion in the Weimer et al. and Arnold et al. literature references (identified above) of AC-coupled PN junction photodiode arrays, capacitive-coupled readout of such devices has not met with success. Readout of PN junction photodiodes is typically accomplished with transfer gate and CCD techniques. As to Schottky photodiode arrays, capacitive readout has not been previously proposed. Instead, various transfer gate and CCD readouts have been employed, as noted above. As a result, the potential advantages of Schottky photodiodes have not been fully realized.
Accordingly, one object of the invention is to provide a high resolution two-dimensional solid-state infrared imaging array.
Another object is to provide a Schottky diode infrared imaging array fabricated on a silicon substrate.
Another object is to provide a highly sensitive infrared imaging array with virtually no inherent crosstalk or lag.
Another object is to provide a Schottky diode two-dimensional imaging array capable of high resolution readout without charge coupled devices.
Another object is to provide a Schottky photodiode array with a high fill factor.